Antisense technology is an effective means for reducing the expression of one or more specific gene products and can therefore prove to be uniquely useful in a number of therapeutic, diagnostic, and research applications. Chemically modified nucleosides are routinely used for incorporation into antisense sequences to enhance one or more properties such as for example nuclease resistance. One such group of chemical modifications includes bicyclic nucleosides wherein the furanose portion of the nucleoside includes a bridge connecting two atoms on the furanose ring thereby forming a bicyclic ring system. Such bicyclic nucleosides have various names including BNA's and LNA's for bicyclic nucleic acids or locked nucleic acids respectively. Various BNA's have been prepared and reported in the patent literature as well as in scientific literature, see for example: Singh et al., Chem. Commun., 1998, 4, 455-456; Koshkin et al., Tetrahedron, 1998, 54, 3607-3630; Wahlestedt et al., Proc. Natl. Acad. Sci. U.S.A., 2000, 97, 5633-5638; Kumar et al., Bioorg. Med. Chem. Lett., 1998, 8, 2219-2222; Wengel et al., PCT International Application WO 98-DK393 19980914; Singh et al., J. Org. Chem., 1998, 63, 10035-10039, the text of each is incorporated by reference herein, in their entirety. Examples of issued US patents and published applications include for example: U.S. Pat. Nos. 7,053,207, 6,770,748, 6,268,490 and 6,794,499 and published U.S. applications 20040219565, 20040014959, 20030207841, 20040192918, 20030224377, 20040143114 and 20030082807; the text of each is incorporated by reference herein, in their entirety.
In one in vivo study with LNA in mice, hepatotoxicity was reported. See, e.g., Swayze et al., Antisense oligonucleotides containing locked nucleic acid improve potency but cause significant hepatotoxicity in animals, Nucl. Acids Res., 2007, 35(2), 687-700.
In another study the induction of exon skipping in cyclophilin A pre-mRNA in HeLa cells compared tricyclo-DNA with LNA (Ittig et al., Nucl. Acids Res., 2004, 32(1), 346-353). In this study the tricyclo-DNA was several fold more effective than the LNA apparently because of its better intracellular distribution even though it had a lower Tm than the LNA. Such data encourage further research into DNA and or LNA analogs.